The present disclosure relates to radio transmitters and receivers in communication networks, especially to it relates to reduction of transmitter leakage in radio base stations.
In wireless communication networks, UEs (User Equipments), communicate data via radio base stations.
In this description, the term “User Equipment” will be used to denote any suitable communication terminal adapted to communicate with a radio base station. A UE may be implemented as a mobile phone, a PDA (Personal Digital Assistant), a handheld computer, a laptop computer, etc. A “radio base station” may be implanted as a NodeB, an eNodeB, a repeater, etc.
In “heterodyne” receiver structures, an incoming RF-signal is fed into a mixer where it is mixed with a local oscillator (LO) frequency. The mixer output is a down-converted version of the RF-signal of an intermediate frequency, where the LO frequency is removed from the incoming RF-signal, i.e. the RF-signal spectrum is transferred into an IF (Intermediate Frequency) spectrum.
In double conversion heterodyne systems, which are common today, a received RF-signal spectrum is frequency converted in two steps before the signal is fed into a receiver arrangement. First the received RF-signal is frequency converted from a RF frequency range into a lower first IF frequency range, and then the first IF frequency range is frequency converted into a second IF frequency range, before being A/D (Analogue-to-Digital) converted and fed into the receiver arrangement.
With reference to FIG. 1, which is a schematic block diagram, an overview of an RBS (radio base station) 100 according to the prior art will now be described.
The RBS 100 is arranged to transmit DL (Downlink) data to a plurality of UEs (User Equipments) located in a coverage of a cell which is served by the RBS 100, according to TDD (Time Division Duplex). The RBS 100 is further arranged to receive UL (Uplink) data from the UEs.
The RBS 100 comprises a transmitter part (TX) 110, a receiver part (RX) 120, and a circulator 140 which alternating connects one of the transmitter part 110 and the receiver part 120 to an antenna 130. The transmitter part 110 is arranged to receive a transmitter baseband TX Bb, frequency converting the transmitter baseband TX Bb into a transmitter RF (Radio frequency) spectrum TX RF to provide to the antenna 130, via the circulator 140. In the transmitter part 110 is further a mixer 114 arranged to frequency convert the transmitter baseband TX Bb into the transmitter RF spectrum TX RF, by mixing the transmitter baseband TX Bb with an output frequency fLO of a local oscillator TX LO of the RBS 100.
Upon reception of a receiver RF spectrum RX RF at the antenna 130, the circulator 140 provides the receiver RF spectrum RX RF to the receiver part 120. First, the receiver RF spectrum RX RF is frequency converted into a receiver IF (intermediate frequency) spectrum RX IF by in a first RX mixer 122 mixing the receiver RF spectrum RX RF with an output frequency of a first local oscillator RX RF LO of the RBS, resulting in the receiver IF spectrum RX IF. Then, the receiver IF spectrum RX IF is frequency converted into a receiver baseband RX Bb, by in a second RX mixer 124 mixing the receiver IF spectrum RX IF with an output frequency of a second local oscillator RX IF LO of the RBS 100.
Typically, transmitter parts and the receiver parts are isolated from each other in RBSs. However, because a sufficient isolation is difficult to achieve, a TX LO leakage signal which originates from the TX LO output signal will reach the receiver part 120. Typically, such a TX LO leakage signal reaches the receiver part 120 via radiation crosstalk within the RBS 100, or via ground connections or power connections. The TX LO leakage signal may further reach the receiver part 120 via the circulator 130.
Thus, there is a problem to prevent a TX LO leakage signal which originates from the transmitter part from affecting the receiver part.
With reference to FIGS. 2a-c, which are schematic diagrams, definitions of RF (Radio Frequency) spectrums will now be described according to the prior art.
In a transmitter part of an RBS, a transmitter RF signal spectrum TX RF will be transmitted. As described above, the transmitter RF signal spectrum TX RF originates from a transmitter baseband spectrum TX Bb which has been frequency converted by being mixed with a transmitter local oscillator signal TX LO. In FIG. 2a, the TX RF signal spectrum and the TX LO signal is shown.
In FIG. 2b, an RX operating band and a receiver RF signal spectrum RX RF are shown. The RX operating band is the available frequency band in which receiver RF signal spectrums RX RFs are possible to receive. The RX operating band is standardised by 3GPP (Third Generation Partnership program) The receiver RF signal spectrum RX RF is set by a network operator, and defines the frequency spectrum that is used for UL (Uplink) communication. In the figure is further the transmitter RF signal spectrum TX RF indicated (dashed line), and also a transmitter leakage signal TX LO, which originates from the transmitter local oscillator signal TX LO of the transmitter part.
As seen in the FIG. 2b, the transmitter local oscillator signal TX LO leakage is not within the receiver RF signal spectrum RX RF and is therefore possible from filtering out by the receiver part.
In FIG. 2c, the same RX operating band, transmitter local oscillator signal TX LO leakage, and transmitter RF signal spectrum TX RF (dashed) as shown in FIG. 2b are illustrated. The FIG. 2c differs from FIG. 2b in that the receiver RF signal spectrum RX RF is different. In this example, the receiver RF signal spectrum RX RF overlaps the frequency of the TX LO signal, and consequently, also the transmitter local oscillator signal TX LO leakage. Due to the overlap, the TX LO leakage signal will not be possible to filter out without affecting the receiver RF signal spectrum RX RF, and therefore, the transmitter local oscillator signal TX LO leakage will disturb the receiver part. Thus, there is also a need to decrease disturbances at UL communication for transceivers operating in TDD mode.